THE PATHOGKNIC BACTERIA. 531 



consider those phenomena, which have been described as 

 bacterial diseases of plants, very briefly and with a certain 

 reserve. This part of the work has been considerably facilitated 

 l)y the use of Ludwig's compilation of bacterial disea.ses/ and by 

 Migula's account of them from the bacteriological point of view. 



Migula considers that only five diseases of plants have been 

 definitely proved to be due to bacteria, namely, pear or apple 

 l)light, sorghum blight, the bacterial disease of the maize, the 

 l)ulb-rot of hyacinths, and the wet-rot of potatoes. We shall, 

 however, indicate l)riefiy some other plant diseases which are 

 suspected to liiive a bacterial origin. The sUme-fiuxes of trees 

 have been already considered along with the genus Fjidonif/crs 

 (p. 141), so that we omit them here. 



Pear and Apple Blight. 



This destructive disease of the apple and i)ear in Xmlli 

 America has been proved by the investigations of liurrill - 

 and Arthur to be, without doubt, of bacterial origin. The 

 disease has been known for over 100 years, and occurs 

 with disastrous effects on fruit-trees • in the orchards, as well 

 as on crabs and other wild species. Pear trees seem to suffer 

 most in the Eastern States, apple trees in Iowa and elsewhere, 

 while none of the species of Pi/rns, Ci/donia, and tSorbus are 

 exempt from attack. 



The disease appears first on the l)ark as little dead spots ; 

 these, however, rapidly enlarge till death of twigs, branches, and 

 even stems may follow. As a result of death of twigs, the 

 leaves turn l)rown and fall, while a dark fiuid exudes from the 

 diseased l)ark. The presence of bacteria has been proved in 

 this exuded sap as well as inside the cells, and infections have 

 been successfully carried out from pure cultures. The name 

 Micrococcus amylovorous was given by liurrill to the organism. 

 It flourishes on the sour unripe fruit, and in the tissues of 



' Lehrbnch der niedereii Krypto<iamen, 1892. 

 Migula, KrUische Uehernicht d. Pflanztnkranklicitfu dnrch Bali i rim n ntr- 

 ^arht, 1892. 



Prillieux et Delacroix. " Maladies baccillaires."' (.'ompteM rendu, 1894. 



-Bun-ill, Thi Amtrkan Xaturndst, 1881. 

 Arthur, Report of New York A>jric. Exper. Station, 1887. 

 Waite, Yearbook of U.S. Dept. of Afjricuitiire, 1895; ilescription ami 



treatment. 



